The research deals with the problems of the nature of the controlling stimulus in discrimination learning, and with the elimination of responding with reinforcement procedures. A number of experiments have shown that the aspect of a stimulus that controls behavior differs from one subject to the next. The present research studies how multiple training sets, stimulus similarity, the type of discrimination situation, and the procedure used to eliminate responding to certain stimuli influence control by different stimulus aspects. It also investigates how reinforcement for not responding in the context of maintained reinforcement for responding influences performance.